In a magnetic disk unit employing a sector servo system, which is generally used today, there is provided in sectors of each cylinder servo information containing a track number and a signal for correcting a position between tracks. When no command comes from a host computer, a magnetic disk unit usually performs position control of its head so that the head is held on a certain track in accordance with servo information in each sector. This state is called an idle state. When a command comes from the host computer in this state, the command must be immediately executed.
When a small magnetic disk unit is used in practice, the proportion of time spent in the idle state is very large. In particular, the power consumption of a magnetic disk unit used in a battery-driven notebook type personal computer presents a great problem. In view of this, to reduce power consumption during the idle state in a recent sector servo type magnetic disk unit, the power for write or read circuitry is turned on only for each servo sector and is turned off for each data sector, the remaining section of each sector. Specifically, the power to the write or read circuitry is switched at a fixed duty ratio so as to reduce the power consumption.
The power off ratio of the write or read circuitry however is limited to about 40 -50%. This is because the duty ratio is limited by the length ratio between the servo sector and the data sector.